Rechnungsdatum und Belegupload ueberarbeiten

prisma/migrations/202605011330_invoice_date/migration.sql

@@ -0,0 +1 @@
+ALTER TABLE "expenses" ADD COLUMN "invoice_date" TIMESTAMP(3);

prisma/schema.prisma

@@ -133,6 +133,7 @@ model Expense {
   recurrenceStartAt DateTime?         @map("recurrence_start_at")
   paidAt         DateTime?         @map("paid_at")
   documentedAt   DateTime?         @map("documented_at")
+  invoiceDate    DateTime?         @map("invoice_date")
   proofUrl       String?           @map("proof_url")
   createdAt      DateTime          @default(now()) @map("created_at")
   updatedAt      DateTime          @updatedAt @map("updated_at")